A lithium ion secondary battery which forms a negative electrode using a material capable of lithium ion storage and discharge can control the deposition of dendrite compared to a lithium battery which forms a negative electrode using metal lithium. As a result, it is possible to provide a battery which has a high capacity and high density of energy which can prevent battery short and improve safety.
In recent years, while further high capacity is being demanded for lithium ion secondary batteries, improvements in large current charging and discharging capabilities due to a reduction in battery resistance are being demanded for a battery for use in power systems. Conventionally, efforts have been made to achieve this by increasing high capacity of a lithium metal oxide positive electrode material or carbon group negative electrode material itself which are battery reactive materials, a reduction in the particle diameter of these reactive material particles, an increase in electrode area using particle specific surface area or battery design and further, a reduction in liquid diffusion resistance using thinner separators. However, this leads to an increase in a binder due to small particle diameter or an increase in specific surface area which adversely affects high capacity, and because positive/negative electrode materials peel from or fall off metal foil, which is an electric collector, causing battery internal shorts, and safety of lithium ion secondary batteries is lost due to a drop in battery voltage resistance or dramatic thermal runaway. Thus, changes in the type of binder used in order to increase binding abilities with a foil were examined in Japanese Laid Open Patent H5-226004.
However, by changing the type of binder, capacity can be increased, but improvement of large current charging and discharging capabilities due to a reduction in resistance of a binder is insufficient, and development for use in electrical power tools or hybrid cars which require large current charging and discharging which is a large function impairment in lithium ion secondary batteries compared to secondary batteries such as a nickel-cadmium battery or nickel-hydride battery was difficult.
In addition, efforts have been made using carbon conductive materials with the goal of reducing electrode resistance with respect to large current charging and discharging of lithium ion secondary batteries in Japanese Laid Open Patent 2005-19399, Japanese Laid Open Patent 2001-126733 and Japanese Laid Open Patent 2003-168429. However, when a charging and discharging cycle using a large current is repeated, a conductivity pass of particles between positive/negative electrodes is lost due to expansion and contraction of a positive/negative electrode material and as a result, a large current rapidly no longer flows which is a problem.
On the other hand, in recent years, olivine-type lithium iron phosphate (LiFePO4) has been attracting attention as a positive electrode material for a lithium ion battery considering the importance of safety and costs. However, this material has a large resistance and reducing this resistance has become a large problem in Japanese Translation of PCT International Publication 2000-509193 and Japanese Laid Open Patent H9-134724.
In order to solve the problem of olivine-type lithium iron phosphate described above, various examinations have been performed to make a positive electrode material by a composite of olivine-type lithium iron phosphate and black carbon which is a conductive material in Japanese Laid Open Patent 2002-75364, Japanese Laid Open Patent 2002-110162, Japanese Laid Open Patent 2004-63386, Japanese Laid Open Patent 2005-123107, Japanese Laid Open Patent 2005-302671 and Japanese Laid Open Patent 2007-80652. The capabilities of a positive electrode material have been improved using an olivine-type lithium iron phosphate as a result of these examinations, however, high capabilities equal to or higher than already existing positive electrode materials for a lithium ion secondary battery have yet to be achieved. Japanese Translation of PCT International Publication 2009-503182 is also referred as a prior art document.